I never did get to play with the TH55 while at the DevCon (I could kick myself now) so I can't comment, but from the short live demo I saw, I could have sworn the handwriting in color was vector-based.

Whatever the case, this is an interesting idea -- designing a PIM for folks that like keeping a messy, scribbled up paper planner. Maybe this will be enough to get paper-bound users over to handhelds?